Generally, a driving circuit of a LED array usually utilizes a critical conduction mode (CRM) or quasi-resonant (QR) buck-boost converter. Such buck-boost converter needs an additional winding for providing a current to a control chip, thereby implementing a zero-current switching function. FIG. 1 shows a conventional driving circuit of the LED array. An alternating current (AC) voltage VACIN is rectified by a rectifier 15 so as to generate a direct current (DC) voltage Vin. A control chip 13 controls a transistor Q1 to be turned on or turned off so as to generate a stable output voltage Vo or a stable output current Io, so that the LED array 11 can be driven. Referring to FIG. 1, a ground terminal of the LED array 11 is different from that of the control chip 13. Accordingly, the control chip 13 is unable to detect the voltage and the current on the LED array 11 directly. Thus, an additional winding N2 is required in order to implement the following functions:
1.) providing a current Ivdd to charge a capacitor CVDD so as to provide a supply voltage to the control chip 13;
2.) zero-current switching function; and
3.) over-voltage protection function of the output voltage.
FIG. 2 is a waveform diagram of the voltages and the currents in the circuitry shown in FIG. 1, in which the waveform 17 represents a voltage Vds of a drain terminal of the transistor Q1, the waveform 19 represents a voltage VAUX on the winding N2, and the waveform 21 represents a current Idout on a diode Dout. The current Ivdd provided by the winding N2 in the circuitry of FIG. 1 will charge the capacitor CVDD, thereby maintaining the supply voltage VDD of the control chip 13. When the transistor Q1 is turned off, the voltage VAUX on the winding N2 is proportional to the voltage Vds of the drain terminal of the transistor Q1, as shown by waveforms 17 and 19. Simultaneously, a voltage on a winding N1 is almost the same as the output voltage Vo. Thus, the voltage VAUX is also proportional to the output voltage Vo. Resistors Rzcd1 and Rzcd2 divide the voltage VAUX to generate the voltage Vd to a pin ZCD of the control chip 13. The control chip 13 is able to judge the value of the output voltage Vo via the voltage Vd, thereby achieving the over-voltage protection of the output voltage Vo. Referring to waveform 21 in FIG. 2, when the current Idout on the diode Dout decreases to zero, the voltage VAUX on the winding N2 generates a resonance, which can be utilized to implement the zero-current switching of the current Idout. However, the additional winding N2 will cause a higher cost and increase the size of the driving circuit board.
Therefore, it is desired a LED driving circuit that needs no additional winding.